It would have been hard to keep the new Doctor a secret for any length of time even years ago, so they didn't even try then.
Now it would be absolutely impossible!
I think they're done pretty well to keep it a secret this time until Sunday night, mind you I haven't seen the Sunday papers yet!

I was just thinking earlier - I remember my reaction to Chris regenerating into David. I had no idea who he was going to be, I was about 12 at the time. All I remember saying is "Oh its him!!" To this day I can't remember what I saw him in to think that, unless it was mistaken identity.

Never looked back, mind. That David fella made me a huge fan of the show.

It'd be against the BBC's interests to keep it a secret right up until the moment the new Doctor would appear on-screen because they need to "sell" the new Doctor to the fans. They'll want to know more about him as an actor, what he'll bring to the role, etc. Steven Moffat and the BBC will need to get the naysayers warmed up to him by the time he takes over the role. Also, like someone else said, it would very likely leak, either by way of a loose-lipped DW production team member or an intrepid fan with a camera waiting for the right moment to spot an actor walking around the set or what not.

No, keeping it a secret would be a terrible idea. After the Christmas special all the discussion and media coverage would be centred around the Twelfth Doctor's reveal rather than Matt Smith's final moments and the Swan Song episode itself like it should be.
Also actually keeping it a secret for that long would be nigh on impossible.

Cos people on tv sets have mobile cameras with phones included. Someone on the set just needs to take out their mobile, point it at the new actor, click - photo taken - and upload it onto a site and the next thing it's spread across the internet and everyone knows about it! In the internet/mobile phone era it's impossible to stop things leaking out.

Instead of a big reveal would it not be better if we find out who the new Doctor is on Christmas day when he regenerates?

We all knew Tennant was going to be 10 and Smith was going to be 11 but what if we had no idea Who would be 12.

So Moffat is supposed to read this thread and have a last minute change of heart? At the end of tonight's live show, he should just tell everyone that he's keeping the new Doctor's identity a secret until the episode is shown?

I'm looking forward to tonight's thread in which some dimwit will declare the announcement of the next Doctor to be a "MASSIVE SPOILER" which Digital Spy shouldn't have allowed in headlines and thread titles. Come on, there's going to be at least one person whose "plan" to avoid the announcement until Christmas Day will be thwarted by their foolish decision to look at the Internet.

I think the BBC have done exceedingly well to keep it a secret up to now, actually.

I still think industry insiders and the press have agreed to play along with any speculation to keep the fans guessing...I suspect a few in media land already know who the new pilot of the Tardis will be.

While this is different and good how they are doing a live program for this a few things don't make sense to me.

They could have kept it a secret until xmas where he to regenerate at the end of the episode as closed studio recording a week or 2 before broadcast of just a single scene could be done if they are set to go in advance.

But then it strikes me that the xmas specials have never really been a main heavy and serious episode and often xmas kind of themed in many respects. So it would never make much sense to have a major event like a regeneration in what is a more lighter less serious episode.

I think the regeneration will be at the end of the 50th special and they want to sign there wanted actor now to film a closed studio scene or 2 to properly finish recording. No reason Smith then couldn't also be in the xmas special to in some form of flashback or to do with the regeneration etc scenes. Makes far more sense to use the 50th for the regeneration than the xmas special.